European patent application EP 0 519 242 A1 describes a wear protection layer for a decorative panel comprising silane coated hard material particles and a compacting agent and a lubricant.
U.S. Pat. No. 3,928,706 discloses a method for producing wear protection coatings consisting of a core paper, a decorative paper, a wearing layer and an overlay paper. The wearing layer, which consists of a thermosetting synthetic resin, a water-insoluble hard material finely dispersed therein having a Mohs hardness of at least 7 and cellulose fibers also finely dispersed therein, is either applied onto a surface of the decorative paper or the overlay paper. All three papers are impregnated with a thermo-setting synthetic resin and are processed in a conventional manner to a single laminate by compressing them at temperatures of about 150° C. between highly polished die plates.
DE-A-196 04 907 describes a method for producing an abrasion resistant sheet, in which the paper in the course of the production process even before the drying process is coated directly with a slurry containing relatively crude, abrasion resistant particles as well as a binder. As hard material particles, for example, silica, alumina, alundum, corundum, emery, spinel and various carbides are cited.
WO 97/00172 describes a method in which an overlay paper for the manufacture of abrasion resistant laminates is coated on both sides with hard material particles.
A disadvantage of the wear protection layers known from the prior art is, however, that the pressing tools during the compressing of the laminate become heavily scratched or worn by the hard material particles contained in the wear protection layer. This occurs both in the highly polished mirror surfaces of the pressure plates used in the discontinuous operation and in the surfaces of the pressing belts used in the continuous operation, such that the pressing tools rapidly become unusable and must be replaced. This leads to significant costs.
EP 1339545 B1 discloses a wear protection layer based on a synthetic resin, which in addition to hard material particles includes spherical solid particles, such as glass beads. A disadvantage in the use of such glass beads is that the decor image can become optically impaired. In particular, an optical impairment caused by the glass beads used can result in problems with respect to the alignment of the texture to be introduced within the surface relative to the decor image.
DE-C-195 08 797 tries to solve the wear problem during the production in that the decorative paper is provided with an abrasion resistant layer, without applying a corresponding prefabricated overlay paper by compression, by adjusting the viscosity of the synthetic resin for the coating of the decorative paper so that the finished decorative paper has a smooth, abrasion-resistant layer from which hard particles no longer protrude. A high viscosity, however, leads to entrapped air and thus a lack of transparency of the layer. The problem of wear of the pressing tools is not solved in this way, because during the final assembly under the usual pressing conditions there is still a contact between the mirror surface of the pressing tool and the hard material.
U.S. Pat. No. 5,344,704 describes a way for reducing the wear of the pressing tools in that pre-cured resin particles are incorporated together with the hard material particles. If these pre-cured resin particles, however, are to protect the pressing tools, they have to be greater than the hard material particles. However, because the resin particles do not have a sufficient hardness, the abrasion resistance of the wearing layer is greatly reduced. If the pre-cured resin particles are of equal or less size than the hard material particles the pressing tools can no longer or only insufficiently be protected. A further disadvantage is that the melamine resins usually used achieve the high transparency required for a high-quality decorative layer only if they are completely cured under pressure. Thus, it appears that both the mechanical strength and the decorative effect are problematic.